slides-07

ACTUATORS

PRINCIPLE

MEDIA

Mechanical structure under load

Mechanics

Gas under pressure

Pneumatics

Fluid under pressure

Hyraulics

Electrons at potential

Electricity

FACTORS TO BE CONSIDERED WHEN

CHOOSING THE TYPE OF MEDIA

•What is the starting torque?

•How much space is available?

•Are the conditions constant or variable?

•What is the typical duty cycle?

•What power level is required?

•What rotary speed is available?

•Is the environment is potentially hazardous?

•How safe is the system to be?

•What finesse of control is required?

•What total efficiency is expected?

•Is the environment unfriendly?

•What is the transmission to cover?

Various single acting cylinders

HYDRAULIC CYLINDERS

Double acting Cylinder

)

(

4

A

d

−

d

π

=

A

Q

=

U

DOUBLE ACTING CYLINDER

A

A

Q

=

U

A

A

Q

A

U

=

=

Q

A

A

Q

A

U

=

=

Q

×

Hydraulic Servovalves

Valves Perform These Basic Functions:

• Change Flow Direction

• Change Flow Rate

Port

Land

Port

Land

Overlapped Land

Underlapped Land

Introduces DEAD ZONE in the

neighborhood of central position of

the spool.

There is a LEAKAGA FLOW even

in fully enclosed case.

HYDRAULIC SERVO VALVE ANALYSIS

k

U

k

P

Q

=

δ

+

δ

δ

k

U

k

P

Q

=

δ

−

δ

δ

Q

Q

+

=

Since flow rate Q

into the actuator increses U increases and decreses P

increases:

Since flow rate Q

from the actuator increses with both U and P

, we have:

Disregard compresibility of the liquid (δQ

= δQ

), assuming the hyraulic

piston is doble acting and areas are equal. Otherwise consider the general

case δQ

≠ δQ

'

HYDRAULIC SERVO VALVE ANALYSIS

U

Q

k

⎟

⎠

⎞

⎜

⎝

⎛

∂

∂

=

P

k

U

k

Q

=

δ

−

δ

δ

Where the flow gain:

Note that the pressure sensitivity is:

U c

P

Q

k

⎟

⎠

⎞

⎜

⎝

⎛

∂

∂

−

=

k

k

P

Q

k

⎟

=

⎠

⎞

⎜

⎝

⎛

∂

∂

=

0

as

δ

→

∂

∂

→

δ

δ

Q

U

P

U

P

P

Q

U

Q

U

P

⎟

⎠

⎞

⎜

⎝

⎛

∂

∂

⎟

⎠

⎞

⎜

⎝

⎛

∂

∂

−

=

⎟

⎠

⎞

⎜

⎝

⎛

∂

∂

We have:

VALVE-CONTROLLED SYSTEMS

Types of Valve Controlled Hydraulic Actuators

•Constant-Pressure Systems

The supply pressure is kept constant at variable flow rates

•Constant-Flow Systems

The supply flow is kept constant at varying pressure levels.

Constant-Flow operation requires a constant-flow pump, which is more

economical than variable-flow pump. Despite this, constant pressure

systems are more commonly used. They are particulary useful when

more than one actuator is powered by the same hydraulic supply.

PUMP-CONTROLLED SYSTEMS

Electric Drive

Variable-Flow

_Pump

Hydraulic

_Motor

Load

Control

Control

Pump-Controlled hydraulic Drives are suitable only when

a single actuator is controlled

Alternative Control methods

include: Controlling the pum stroke

or speed of AC motor

HYDRAULIC ACUMMULATORS

WHY HYDRAULIC ACCUMULATORS ARE NECESSARY?

• Compansating for peak flows and pressures by storing energy and thus

economizing the pump size

• Providing a stand-by power source in case of emergencies such as

pump failure or fast shut down

•Compensating for leakage losses

•Damping out of system surges, shocks and vibration

•Reducing pump ripple effects

•Compensating volume changes due to temperature and pressure

variations

Common Accumulator Types

•Gas-Charged Accumulators

–Piston

–Bladder (closed bag)

–Bladder (open bag)

–Diaphram

•Loaded Accumulators

–Spring-Loaded

–Weight-Loaded

Gas-Charged Accumulator Types

Fluid

spring

Piston

Reservoir

Cylinder

_Fluid

Piston

Reservoir

Cylinder

Loaded Accumulator Types

Comparison of Characterisitics

Low

Low

Spring-Loaded

0.07-5

to 50

Diaphragm

0.13-4

(*)

to 25

Bladder (open

bag/open top)

0.16-54

to 69

Bladder (closed bag)

0.2-500

0.5-150

Piston

Variable

10.5

Weight-Loaded

Volume (Liters)

Working Pressure (MPa)

Type